1. Field of the Invention
The present invention relates to a developing apparatus that is used for a copier that employs an electrophotographic system, a laser beam printer, a facsimile machine, or a printing apparatus.
2. Related Background Art
There is available a conventional, well known image forming apparatus wherein an electrostatic latent image bearing member is uniformly charged and an electrostatic latent image is formed on the bearing member by analog exposure, or image exposure, using a semiconductor laser or an LED; wherein the electrostatic latent image is visualized as a developer image by a developing apparatus, and the developer image is transferred to a transfer material; wherein the transfer material is separated from the electrostatic latent image bearing member and is transmitted to a fixing unit; and wherein the developer image is fixed to the transfer material by the fixing unit and the obtained transfer material is output as an image.
The image forming processing will now be described while referring to FIG. 14.
FIG. 14 is a cross-sectional view of the essential portion of an image forming apparatus. The image forming apparatus includes a photosensitive drum 1 as an electrostatic latent image bearing member. A photoconductive layer, such as OPC or a-Si, is deposited on the surface of the photosensitive drum 1 and the drum 1 is rotated in the direction indicated by an arrow A. The surface of the photosensitive drum 1 is uniformly charged to -700 V, for example, by a primary charger 3. Then, image exposure process 12 is performed in accordance with image signal information to attenuate to -200 V, for example, the surface potential of the exposed portion of the photosensitive drum 1, and to form thereon a latent image consonant with an image signal. It should be noted that a semiconductor laser or an LED array is used for the image exposure process 12.
The latent image is developed by a developing unit 2, a dry, one-component developing unit, and is visualized as a toner image. Since the developing unit 2, which uses the dry, one-component developer, is simple and no carrier replacement is required, its durability is excellent and it has a long service life. As the development method, jumping development, which uses a one-component magnetic toner, for example, is employed.
The developing unit 2 employs black, negatively charged toner that, following the application, during the development process, to the developer bearing member of a DC bias of about -500 V as a developing bias, is used to develop a reversed latent image that is visualized as a toner image. The pre-transfer process (normally performed in conjunction with a DC or an AC corona application or a charge elimination process) is performed using a charger 10, as needed, and the toner image is transferred by a transfer charger 4 from the photosensitive drum 1 to a transfer material adhering thereto. The transfer material bearing the toner image is then transmitted to a fixing unit 7, which fixes the toner image to the transfer material. In this manner, a desired image is obtained. Thereafter, residual toner after transfer on the photosensitive drum 1 is removed by a cleaning unit 6, so that it is ready for the next image forming sequence.
Relative to increasing the processing speed of the image forming apparatus, since the developing unit employs a one-component developer and is rotated counterclockwise to limit the thickness of a layer, toner is rubbed heavily, so that even for the same toner the triboelectricity downstream is considerably higher than that upstream. Therefore, image development performed by the developing sleeve upstream differs from that performed by the developing sleeve downstream, so if a common developing bias is employed to reduce space and manufacturing costs, the development performance differs for each developing sleeve, making density control difficult. This phenomenon is contrary to the need for gradation (tone) of image and stability of the gradation that can cope with current digital images and graphics, and is especially apparent when a one-component magnetic toner is employed for which the strength of the charge applied is small and the development is poor.
With a sleeve that is not coated with SUS and the like, for example, a sleeve ghost image will occur, whereafter an image will appear at a sleeve pitch. Further, for a fast, image-forming apparatus, for which development time is shortened due to its high processing speed, and since a satisfactorily long development time is not provided, the development process is unstable and density fluctuation is increased.